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New Tool Cheaply, Efficiently Captures Nitrates

When Iowa State University researchers asked Justin Hanson about installing a saturated buffer strip on his farm near Roland, Iowa, his first concern, naturally, was if it would hurt his nearby crops. “First thing I asked, is it going to back up my tile?” he recalls.

Since its installation in late 2009, it hasn’t. The 120-acre field next to it is rotated between corn and soybeans. It has about 11 tiled acres at a flatter top of a slope that drains into Bear Creek. Hanson’s family installed a riparian buffer of switchgrass, shrubs, and trees along the creek in 1995 that’s about 66 feet wide.

Since 2009, the tile no longer goes through a direct outlet into the creek. It hits a control box at the bottom of the hill that diverts water into about 1,100 feet of 4-inch perforated tile running parallel to the creek about 2 feet below the surface. That raises the water table under the buffer, allowing organic matter and anaerobic microbes in the soil to convert nitrates in the tile water back into atmospheric nitrogen. The tile water seeps into the creek through the soil.

In a wet spring, if the flow is too much for the saturated soil to handle, the control box is designed so that tile flow automatically goes through the old tile outlet to the stream. “To me, that’s the out,” says Hanson, who likes the system well enough that he’s having another one installed upstream in a new buffer that will be mainly switchgrass.

Hanson’s field remains as productive as any other in north central Iowa’s rich soils. It hasn’t changed much since 2009. The buffer strip, however, gets frequent visits from a small army of researchers who take water samples in the field and closer to Bear Creek. They measure nitrate levels as well as study nitrous oxide released in the field and the buffer. Nitrous oxide is a greenhouse gas 300 times more potent than carbon dioxide. It’s one of 14 intensely studied buffers in Iowa, Minnesota, Illinois, and Indiana.

The bottom line: Hanson’s saturated buffer works well and is less expensive than other methods of catching unused nitrates at the edge of a field. When water is diverted through the buffer, it removes all of the nitrates in the tile water at half the cost of constructed wetlands and a third of the cost of fall planted cover crops, according to research led by Dan Jaynes, a soil scientist with USDA’s Agricultural Research Service, and Tom Isenhart, an ISU water resources specialist.

Researchers have measured nitrates in the field, halfway through the buffer and near the creek. In the field, nitrate levels are typically 12 parts per million to 15 ppm (above the 10 ppm standard for drinking water). In the wells halfway through the buffer, the level is usually about 5 ppm.

“By the time you get to the stream, it’s all gone,” Jaynes says of the nitrates. So far, early evidence suggests the system doesn’t increase nitrous oxide emissions, either.

For about two years, saturated buffers have been eligible for Natural Resources Conservation Service cost share in several Midwest states. Hanson didn’t have to pay for the research project, but its cost was $2,508 for the tile installed. The control box was another $1,120. Design work runs about $100. Depreciation over 20 years at 4% interest adds about $1,460. The total cost is $5,188 over 20 years, or $259 annually.

Good as they are, saturated buffers have limitations. They can’t be used year-round. “We capture 35% to 40% of the water. You can’t take it all in the spring,” Jaynes says.

Nor can every existing buffer be saturated. In a flat landscape, it couldn’t be used much without backing up tiles, making it ineffective. Along waterways with high, steep banks, a seepage face could cause bank sloughing and erosion. Hanson’s field, it turns out, is ideal, with the riparian buffer sloping to the stream and the field a few feet higher than the buffer. All in all, Jaynes estimates that one out of five of the state’s 380,000 acres in buffers is suitable for the practice. Still, that could remove 32 million pounds of nitrates from Iowa streams a year, the equivalent to about 5% of the current load in Iowa streams.

A saturated buffer is the latest, greatest tool for catching the nitrogen that even the best in-field practices can miss. Other edge-of-field options include bioreactors, prairie strips, and drainage control for tile. Bioreactors also use carbon wood chips buried in a trench where tile water is diverted. They can cost $10,000 to $14,000, and the chips must be replaced after about 15 years.

NRCS EQIP funds can be used to pay up to about half the cost of a control box and tile in states with Interim Conservation Practice Standard #739, “Vegetated Subsurface Drain Outlet.” Iowa and Minnesota have it; other Midwest states may have it soon.

Some state and private programs can replace or add to that cost share. Check with your local NRCS office for details.

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